Internet Engineering Task Force (IETF) F. Zhang
Request for Comments: 7150 Huawei
Category: Standards Track A. Farrel
ISSN: 2070-1721 Juniper Networks
March 2014
Conveying Vendor-Specific Constraints in the PathComputation Element Communication Protocol
Abstract
The Path Computation Element communication Protocol (PCEP) is used to
convey path computation requests and responses both between Path
Computation Clients (PCCs) and Path Computation Elements (PCEs) and
between cooperating PCEs. In PCEP, the path computation requests
carry details of the constraints and objective functions that the PCC
wishes the PCE to apply in its computation.
This document defines a facility to carry vendor-specific information
in PCEP using a dedicated object and a new Type-Length-Variable that
can be carried in any existing PCEP object.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7150.
Zhang & Farrel Standards Track [Page 1]

RFC 7150 Vendor-Specific Constraints in PCE March 2014
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
1. Introduction
A Path Computation Element (PCE) is an entity (component,
application, or network node) that is capable of computing a network
path or route based on a network graph and applying computational
constraints. An architecture for the use of PCEs is defined in
[RFC4655].
The Path Computation Element communication Protocol (PCEP) is defined
in [RFC5440] to exchange path computation requests and responses
between Path Computation Clients (PCCs) and PCEs. It is also used
between cooperating PCEs.
Path computations performed by a PCE depend on a set of constraints
indicated by the PCC. These constraints include the endpoints of the
path to compute (source and destination) and may include other simple
constraints such as bandwidth requirements and metric maxima (for
example, a maximum threshold for the hop count or the Traffic
Engineering (TE) metric of the computed path).
The PCE also needs to use an objective function to qualify the path
it selects as meeting the requirements of the PCC. The PCE may have
a default objective function, but the PCC can also indicate which
objective function it wants applied by placing an Objective Function
object in the path computation request message [RFC5541]. A core set
of objective functions to be supported in PCEP messages is defined in
the base PCEP requirements [RFC4657], and [RFC5541] defines each of
these functions as an abstract formula.
The registry of codepoints used to indicate objective functions is
managed by IANA and new assignments can be made according to "IETF
Review" and "First Come First Served" policies [RFC5226]. PCE
implementations may also choose to offer proprietary, vendor-specific
Zhang & Farrel Standards Track [Page 2]

RFC 7150 Vendor-Specific Constraints in PCE March 2014
objective functions, and there is scope for this within the codepoint
registry created by [RFC5541] using the codepoints that are flagged
as "Reserved for Private Use".
Proprietary objective functions may operate on non-standard
constraints or metrics. The PCEP METRIC Object defined in [RFC5440]
has scope for the definition of new, standardized metrics, but no
facility for the definition of vendor-specific metrics. At the same
time, there is no mechanism in PCEP for carrying other, more complex,
vendor-specific information.
This document defines a new PCEP object, the Vendor Information
object that can be used to carry arbitrary, proprietary information
such as vendor-specific constraints.
This document also defines a new PCEP TLV, the VENDOR-INFORMATION-TLV
that can be used to carry arbitrary information within any PCEP
object that supports TLVs.
It should be noted that by the very definition of "vendor-specific",
the inclusion of either a Vendor Information object or the VENDOR-
INFORMATION-TLV implies an inability to interoperate at a functional
level with implementations from other vendors unless there is some
cooperation agreement between vendors. Sections 2.1 and 3.1 discuss
backward compatibility, which indicates how these protocol constructs
are handled by implementations that do not support them at all, while
text in Sections 2 and 3 describe how implementations handle the
constructs if they understand them, but do not support the embedded
Enterprise Number that indicates to which vendor the constructs
apply.
When vendor-specific information is used by an implementation, the
vendor is encouraged to document the meaning of the information to
encourage wider use and implementation. In particular, when there is
more general interest in a vendor-specific extension, the vendor is
encouraged to bring it to the IETF for standardization as a regular
protocol construct moving it out of the vendor-specific space.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
Zhang & Farrel Standards Track [Page 3]

RFC 7150 Vendor-Specific Constraints in PCE March 20142. Procedures for the Vendor Information Object
A PCC that wants to convey proprietary or vendor-specific constraints
or metrics to a PCE does so by including a Vendor Information object
in the PCReq message. The contents and format of the object are
described in Section 4, but it is important to note that the object
includes an Enterprise Number that is a unique identifier of an
organization responsible for the definition of the content and
meaning of the object.
A PCE that receives a PCReq message containing a Vendor Information
object MUST act according to the P flag in the object header. That
is, if the P flag is set, the object will be treated as mandatory and
the request will either be processed using the contents of the object
or be rejected as defined in [RFC5440] (see also Section 2.1). If
the P flag is clear, then, as defined in [RFC5440], the object may be
used by the PCE or may be ignored. The PCC sets the P flag according
to how it wishes the request to be processed.
The PCE determines how to interpret the information in the Vendor
Information object by examining the Enterprise Number it contains.
An implementation that supports the Vendor Information object, but
receives one carrying an Enterprise Number that it does not support
MUST act according to the P flag in the object. That is, if the P
flag is set, the PCE MUST reject the PCReq as defined in [RFC5440] by
sending an Error message with Error-Type="Not supported Object" along
with the corresponding Vendor Information object.
The Vendor Information object is OPTIONAL in a PCReq message.
Multiple instances of the object MAY be used on a single PCReq
message, and each MUST be treated according to its P-bit setting.
Different instances of the object can have different Enterprise
Numbers.
The object can be present in the PCReq message to enable it to apply
to a single path computation request or to a set of synchronized
requests. This usage mirrors the usage of the Objective Function
object [RFC5541]. Thus, the PCReq message based on [RFC6006] is
encoded as follows using the syntax described in [RFC5511].
Zhang & Farrel Standards Track [Page 4]

RFC 7150 Vendor-Specific Constraints in PCE March 2014
Thus, the PCRep is encoded as follows:
<PCRep Message> ::= <Common Header>
<response>
<response> ::= <RP>
[<vendor-info-list>]
[<end-point-path-pair-list>]
[<NO-PATH>]
[<attribute-list>]
where:
<end-point-path-pair-list> ::=
[<END-POINTS>]
<path>
[<vendor-info-list>]
[<end-point-path-pair-list>]
<path> ::= (<ERO>|<SERO>) [<path>]
<attribute-list> ::= [<OF>]
[<LSPA>]
[<BANDWIDTH>]
[<metric-list>]
[<IRO>]
2.1. Backward Compatibility for the Vendor Information Object
A legacy implementation that does not recognize the Vendor
Information object will act according to the procedures set out in
[RFC5440]. If the P flag is set in the object, the message will be
rejected using a PCErr message with an Error Type of 3 ("Unknown
Object"). If the P flag is not set, the object can safely be ignored
by the recipient.
3. Procedures for the Vendor Information TLV
The Vendor Information TLV can be used to carry vendor-specific
information that applies to a specific PCEP object by including the
TLV in the object.
The PCE determines how to interpret the Vendor Information TLV by
examining the Enterprise Number it contains. If the Enterprise
Number is unknown to the PCE, it MUST treat the Vendor Information
TLV as an unknown TLV and handle it as described in [RFC5440] (see
also Section 3.1).
Zhang & Farrel Standards Track [Page 6]

RFC 7150 Vendor-Specific Constraints in PCE March 2014
Further specifications are needed to define the position and meaning
of the Vendor Information TLV for specific PCEP objects.
3.1. Backward Compatibility
A legacy implementation that does not recognize the Vendor
Information TLV in an object will act according to the procedures set
out in [RFC5440]. As described in Section 7.1 of [RFC5440],
unrecognized TLVs MUST be ignored.
4. Protocol Elements
The Vendor Information object and TLV conform to the format for PCEP
objects and TLVs defined in [RFC5440].
VENDOR-INFORMATION Object-Class 32
VENDOR-INFORMATION Object-Type 1
VENDOR-INFORMATION-TLV Type 7
The format of the VENDOR-INFORMATION object and the format of the
VENDOR-INFORMATION-TLV are the same and are as shown in Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enterprise Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Enterprise-Specific Information ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1 : Format of the Vendor Information Object and TLV
Enterprise Number
A unique identifier of an organization encoded as a 32-bit
integer. Enterprise Numbers are assigned by IANA and managed
through an IANA registry [RFC2578].
Enterprise-Specific Information
The detailed enterprise-specific constraint information carried by
the object. The format and interpretation of this information is
a matter for the enterprise identified by the Enterprise Number.
Such formats and interpretation may be published by the enterprise
Zhang & Farrel Standards Track [Page 7]

RFC 7150 Vendor-Specific Constraints in PCE March 2014
(possibly through an Informational RFC or through commercial
documentation) so that PCCs or PCEs that are not part of the
organization can use the information.
5. IANA Considerations
IANA maintains a registry of PCEP parameters called the "Path
Computation Element Protocol (PCEP) Numbers".
5.1. New PCEP Object
IANA has made an allocation from the "PCEP Objects" subregistry as
follows.
Object-Class Value Name Reference
32 VENDOR-INFORMATION [RFC7150]
Object-Type
0: Unassigned
1: Vendor-Specific Constraints [RFC7150]
2-255: Unassigned
5.2. New PCEP TLV
IANA has made an allocation from the "PCEP TLV Type Indicators"
subregistry as follows.
Value Description Reference
7 VENDOR-INFORMATION-TLV [RFC7150]
6. Management Considerations
This section follows the guidance of [RFC5706] and [RFC6123].
6.1. Control of Function and Policy
A PCEP implementation SHOULD allow configuring of various parameters
as described in [RFC5440]. A PCC implementation that uses vendor-
specific information MAY make the use of this information
configurable either across the whole PCC, per PCE that the PCC uses,
or per path computation request. A PCE that supports vendor-specific
information MAY make the support of this information configurable,
and MAY allow configuration of policies for the use of the
information.
Zhang & Farrel Standards Track [Page 8]

RFC 7150 Vendor-Specific Constraints in PCE March 20146.2. Information and Data Models
A PCEP MIB module is defined in [PCE-MIB] that describes managed
objects for modeling of PCEP communications.
It is NOT RECOMMENDED that standard MIB modules be extended to
include detailed information about the content of the Vendor
Information object or TLV. However, the standard MIB module MAY be
extended to report the use of the Vendor Information object or TLV
and the Enterprise Numbers that the objects and TLVs contain.
6.3. Liveness Detection and Monitoring
This document makes no change to the basic operation of PCEP, so
there are no changes to the requirements for liveness detection and
monitoring set out in [RFC4657] and [RFC5440].
6.4. Verifying Correct Operation
This document makes no change to the basic operation of PCEP, so
there are no changes to the requirements or techniques for monitoring
the correct operation of the protocol out in [RFC4657] and [RFC5440].
Note that "correct operation" in this context refers to the operation
of the protocol itself and not to the operation of the computation
algorithms which are out of scope for all PCEP work.
Mechanisms for verifying the correct operation of computation
algorithms might involve comparing the results returned by more than
one PCE. Scope for this might be limited by the use of vendor
information unless multiple PCEs support the same set of vendor
information.
6.5. Requirements on Other Protocols and Functional Components
This document does not place any new requirements on other network
components or protocols. However, it may be beneficial to consider
whether a PCE should advertise the Enterprise Numbers and vendor
information it supports. This advertisement could be within PCE
Discovery [RFC5088] [RFC5089] or through extensions to PCEP
[RFC5440].
Extensions for discovery and advertisement are outside the scope of
this document.
Zhang & Farrel Standards Track [Page 9]

RFC 7150 Vendor-Specific Constraints in PCE March 20146.6. Impact on Network Operation
The availability of vendor information in PCEP messages may
facilitate more complex and detailed path computations that may
enhance the way in which the network is operated.
On the other hand, the presence of additional vendor-specific
information in PCEP messages may congest the operation of the
protocol especially if the PCE does not support the information
supplied by the PCC. Thus, a PCC SHOULD monitor the capabilities of
a PCE either by discovery mechanisms as described in Section 6.5 or
through the receipt of negative responses. A PCC SHOULD NOT include
vendor information in a PCReq message to a PCE that it believes does
not support the information and that will not forward the request to
some other PCE that does support the information.
7. Security Considerations
The protocol extensions defined in this document do not substantially
change the nature of PCEP. Therefore, the security considerations
set out in [RFC5440] apply unchanged. Note that further security
considerations for the use of PCEP over TCP are presented in
[RFC6952].
Operators should note that an attack on PCEP may involve making PCEP
messages as large as possible in order to consume bandwidth and
processing power. The Vendor Information object and TLV may provide
a vector for this type of attack. It may be protected against by
using the authentication and integrity procedures described in
[RFC5440].
8. References8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5440] Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
March 2009.
[RFC5511] Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax
Used to Form Encoding Rules in Various Routing Protocol
Specifications", RFC 5511, April 2009.
Zhang & Farrel Standards Track [Page 10]